posted by Lisa .
evaluate the trigonometry function using its period as an aid cos(-7pi/3)
7/3 = 2 1/3
2 pi is a full circle
so we have gone around clockwise two circles and pi/3
so we really want cos (-pi/3)
draw a picture of that,60 degrees below the x axis in quadrant 4
the cosine is cos -60 degrees which is 1/2